1. Field of the Invention
The present invention pertains to the field of x-ray sources and amongst other things to targets for x-ray sources.
2. Background of the Invention
In conventional x-ray sources, x-ray radiation is produced by colliding an accelerated stream of charged particles (e.g., electrons) into a solid body. This solid body is often referred to as a "target" or "target assembly." In general, x-rays are produced from the interaction between the energy of the fast moving electrons and the structure of the atoms of the target assembly material. X-rays radiate in all directions from the area on the target assembly where the collisions take place.
"Transmission" targets are employed in x-ray sources in which the useful x-rays are taken from the opposite side of the target from the incident electron stream. This is in contrast to "reflective" targets, in which the useful x-rays are taken from the same side of the target as the incident electron stream.
A significant effect of the x-ray generation process is the production of heat at the target assembly when electrons decelerate within the target assembly material. In conventional x-ray sources, the majority of the incident energy of the electrons is dissipated as heat within the target assembly, while only a relatively small percentage of the incident energy results in the emission of x-rays. If the electron stream is directed at the target assembly as a tightly focussed beam of electrons, high temperatures are generated at a relatively small spot size on the target assembly.
The power handling characteristics of x-ray sources are often limited by the ability of the target assembly to dissipate heat generated at the area of impact of an electron beam. The load that can be safely handled by a particular x-ray source is typically limited by the specific materials forming the x-ray source target assembly and is a function of the heat energy produced during the exposure of the target assembly to the electron beam. The target assembly materials may suffer significant damage (e.g., the target assembly materials may melt or vaporize) if the heat limit of the target assembly materials is exceeded. Factors that affect the amount of heat that can be absorbed without damage include the total area of the target assembly material bombarded by the electron beam, the energy and power of the electron beam employed, the duration of exposure, as well as the melting point of particular target assembly materials.
The particular materials employed in a target assembly play an important factor in determining how much x-ray radiation will be produced by a given stream of electrons. The amount of x-rays produced by the x-ray generating material of a target assembly is a function of the atomic number of the x-ray generating material. In general, materials having a high atomic number are more efficient at x-ray production than materials having lower atomic numbers. However, many high atomic number materials have low melting points, making them generally unsuitable in an x-ray target assembly. Many low atomic materials have good heat-handling characteristics, but are less efficient for the production of x-rays. Tungsten has been commonly employed as a x-ray generating material because of its combination of a high atomic number (Z=74 ), as well as its relatively high melting point (3370.degree. C.).
A transmission target assembly is typically formed with a thin layer of x-ray generating material supported by a substrate made from a material that is relatively transmissive to x-rays. The x-ray generating material is typically a relatively thin layer to minimize self-absorption of the generated x-rays. The substrate material used to support the target material is normally formed from a relatively x-ray transmissive material to avoid attenuating the generated x-rays. In general, a low atomic number material is desirable for use as the substrate material because of its x-ray transmissiveness characteristics. However, such materials typically have a lower melting point than the higher-atomic number materials used for the x-ray producing layer. Because of the transfer of heat from the x-ray generating material to the supporting substrate, the maximum allowable temperature of the transmission target assembly is often limited by the choice of the substrate material rather than the x-ray generating material.
Accordingly there is a need for an x-ray target assembly that is efficient for the production of x-rays, but is capable of withstanding the heat generated from being bombarded with a high power electron beam.